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For more than half a century, pericyclic reactions have played an important role in advancing our fundamental understanding of cycloadditions, sigmatropic shifts, group transfer reactions, and electrocyclization reactions. However, the fundamental mechanisms of photochemically activated cheletropic reactions have remained contentious. Here we report on the simplest cheletropic reaction: the [2+1] addition of ground state 18 O-carbon monoxide (C 18 O, X 1 Σ + ) to D2-acetylene (C 2 D 2 ) photochemically excited to the first excited triplet (T1), second excited triplet (T2), and first excited singlet state (S1) at 5 K, leading to the formation of D2- 18 O-cyclopropenone (c-C 3 D 2 18 O). Supported by quantum-chemical calculations, our investigation provides persuasive testimony on stepwise cheletropic reaction pathways to cyclopropenone via excited state dynamics involving the T2 (non-adiabatic) and S1 state (adiabatic) of acetylene at 5 K, while the T1 state energetically favors an intermediate structure that directly dissociates after relaxing to the ground state. The agreement between experiments in low temperature ices and the excited state calculations signifies how photolysis experiments coupled with theoretical calculations can untangle polyatomic reactions with relevance to fundamental physical organic chemistry at the molecular level, thus affording a versatile strategy to unravel exotic non-equilibrium chemistries in cyclic, aromatic organics. Distinct from traditional radical–radical pathways leading to organic molecules on ice-coated interstellar nanoparticles (interstellar grains) in cold molecular clouds and star-forming regions, the photolytic formation of cyclopropenone as presented changes the perception of how we explain the formation of complex organics in the interstellar medium eventually leading to the molecular precursors of biorelevant molecules. 

Geminal diols—organic molecules carrying two hydroxyl groups at the same carbon atom—have been recognized as key reactive intermediates by the physical (organic) chemistry and atmospheric science communities as fundamental transients in the aerosol cycle and in the atmospheric ozonolysis reaction sequence. Anticipating short lifetimes and their tendency to fragment to water plus the aldehyde or ketone, free geminal diols represent one of the most elusive classes of organic reactive intermediates. Here, we afford an exceptional glance into the preparation of the previously elusive methanediol [CH 2 (OH) 2 ] transient—the simplest geminal diol—via energetic processing of low-temperature methanol–oxygen ices. Methanediol was identified in the gas phase upon sublimation via isomer-selective photoionization reflectron time-of-flight mass spectrometry combined with isotopic substitution studies. Electronic structure calculations reveal that methanediol is formed via excited state dynamics through insertion of electronically excited atomic oxygen into a carbon–hydrogen bond of the methyl group of methanol followed by stabilization in the icy matrix. The first preparation and detection of methanediol demonstrates its gas-phase stability as supported by a significant barrier hindering unimolecular decomposition to formaldehyde and water. These findings advance our perception of the fundamental chemistry and chemical bonding of geminal diols and signify their role as an efficient sink of aldehydes and ketones in atmospheric environments eventually coupling the atmospheric chemistry of geminal diols and Criegee intermediates. 

NASA's New Horizons mission unveiled a diverse landscape of Pluto's surface with massive regions being neutral in color, while others like Cthulhu Macula range from golden-yellow to reddish comprising up to half of Pluto's carbon budget. Here, we demonstrate in laboratory experiments merged with electronic structure calculations that the photolysis of solid acetylene – the most abundant precipitate on Pluto's surface – by low energy ultraviolet photons efficiently synthesizes benzene and polycyclic aromatic hydrocarbons via excited state photochemistry thus providing critical molecular building blocks for the colored surface material. Since low energy photons deliver doses to Pluto's surface exceeding those from cosmic rays by six orders of magnitude, these processes may significantly contribute to the coloration of Pluto's surface and of hydrocarbon-covered surfaces of Solar System bodies such as Triton in general. This discovery critically enhances our perception of the distribution of aromatic molecules and carbon throughout our Solar System.